Transfer RNA is central to translation and removal of the 3' trailer is central to tRNA maturation. tRNase Z, the pre-tRNA 3' processing endonuclease, is encoded in a short form (tRNase ZS) in bacteria and archaea and in a long form (tRNase ZL) found only in eukaryotes. As a functioning homodimer, tRNase ZS requires an extensive dimer interface for the subunits to reliably find each other. tRNase ZL originated as a tandem duplication of tRNase ZS which freed the amino and carboxy domains of tRNase ZL to evolve semi-independently, contributing to the ~1,700X greater catalytic efficiency of human tRNase ZL over tRNase ZS. tRNase ZL purifies and presumably functions as a monomer. The amino and carboxy domains of tRNase ZL are linked by a ~70 residue flexible tether. Tethered domains arising from tandem duplications of homodimer proteins may be common and tRNase ZL illustrates the benefits of this evolutionary theme. Flexibility of the tether could also explai the lack of success at crystallizing tRNase ZL. We propose to express and crystallize the amino and carboxy domains of tRNase ZL +/- tether and solve their structures. Success with the domains will guide crystallization of full length tRNase ZL. Complementing domain strategy, formation of a tRNase ZL-tRNA complex could stabilize flexible regions, allowing crystallization of full length tRNase ZL. PUBLIC HEALTH RELEVANCE: tRNase Z has been suggested to be a human prostate cancer susceptibility gene. Numerous pathogenesis-related mutations in human mitochondrial tRNAs affect tRNA metabolism including the tRNase Z reaction. The tandem duplication of tRNase ZS that gave rise to tRNase ZL, including the flexible tether, is an important motif in evolution and enzymology.